i had the same problem with the one i just purchased. after huge trials and errors and working systematically from the airbox backwards, i discovered that it was a huge vacuume leak!! finally after the 27 years that the bike had been alive , the gaskets inbetween the inlet manifolds and the head rotted out!!!. quick clean, and new gaskets (which u can still buy genuine, manifolds too!!) bolted her back together and hey presto!!!! i had a rocket again.